1. Field of the Invention
The present invention relates to a semiconductor device that operates a voltage driven digital transistor, the kind that operates when a certain voltage or higher is applied to the base of a bipolar transistor, and more particularly relates to a semiconductor device that operates a digital transistor constructed such that a zener diode is built into a base, the operating voltage can be adjusted to the desired level, and the transistor will not break down even when a large current is applied.
2. Description of the Related Art
FIG. 6 illustrates the fundamental structure of a bipolar transistor used in the past. A p-type base region 12 is formed at the top part of an n-type semiconductor layer 11, and an n-type emitter region 13 is formed inside this p-type base region 12. A base electrode 16 is joined to the base region 12 via a contact region 14 of a p+-type, and an emitter electrode 17 is joined to the emitter region 13. A collector electrode 18 is provided on the back of a lower n+-type semiconductor substrate 11a of the n-type semiconductor layer 11. The numeral 19 indicates an insulating film.
When a bipolar transistor structured such as this is used to produce a voltage driven transistor such as a digital transistor, as shown in FIG. 7, for example, the circuit is configured so that the transistor Q is switched on and a collector current IC flows corresponding to a base current IB when a power supply voltage VCC is applied between the collector and emitter through a resistor R1 and a specific voltage is applied to the base of a bipolar transistor Q through split resistors R2 and R3, but the transistor Q is not switched on when the specified voltage applied to the base is not attained.
As mentioned above, a digital transistor is an element in which a circuit that uses a base current to raise the potential between the base and the emitter to the voltage at which a transistor Q operates (0.7 V or higher) is formed on a single chip, but since current flows to the split resistors R2 and R3 up to the point when the transistor Q is switched on, the current amplification hFE has poor linearity, as shown by the broken line Q in FIG. 4, and the base current IB has to be increases in order to increase the collector current IC. FIG. 8 illustrates the relation between the collector current IC and the saturation operating voltage (VCE(sat)) between the collector and emitter, and it can be seen that the base current IB has to be sufficiently large in order to lower the saturation voltage between the collector and emitter as well.
Furthermore, the input voltage at which the transistor switches on can range anywhere from 1.5 V to several dozen volts depending on the user, and the current can also vary considerably with the user, so the split resistors R2 and R3 have to be manufactured in various combinations of about 1 kΩ to 100 kΩ, and the problem with this is that not only manufacturing, but maintaining inventory also becomes extremely complicated.
Also, with the transistor structure discussed above, the p+-type contact region 14 for obtaining ohmic contact is formed by raising the impurity concentration at the connection between the base region and the base electrode 16, but if the impurity concentration is high in this contact region, the electrons that serve as minority carriers will be blocked by the p/p+ junction between the base region 12 and the contact region 14, meaning that electrons will accumulate in the base region 12 during the switching operation. This results in greater switching loss and hinders high-speed switching (the off time in particular becomes longer), and also leads to higher power consumption.
Furthermore, a problem with the bias setting resulting from the above-mentioned split resistors is that the speed is reduced by such factors as the load capacity of the resistors. Also, noise tends to be picked up if the zener diode is connected externally to the base of the transistor, which necessitates noise elimination by connecting a capacitor in parallel, and the capacity generated by connector leads and so forth also lowers the speed.
Meanwhile, in order to obtain a bipolar transistor with faster transistor switching and lower power consumption, as shown by the cross sectional structure and the equivalent circuit thereof in FIG. 5, the inventors invented a transistor structured such that a diffusion region 24 of a different conductivity type from that of the base region (the same conductivity type as in the emitter region) is formed at the contact of the base electrode 16, and the base electrode 16 is contacted with this diffusion region 24, and this invention was disclosed in U.S. patent application Ser. No. 09/873412. The result of this structure is that the diffusion region 24 becomes a reverse diode toward the sections of the base region 12, and this transistor has a zener diode ZD built into the base as shown in FIG. 5b. Accordingly, the transistor Q is not switched on unless an input voltage over the zener voltage of this zener diode is applied, and can be made to operate as a digital transistor by adjusting this zener voltage.
However, the concentration of the diffusion region 24 has to be adjusted in order to adjust the zener voltage of the zener diode structured as above, but even though it is preferable in terms of the process for the emitter region 13 to be formed at the same time, because of the characteristics required of the transistor, the concentration of the emitter region cannot be freely changed, and even if this diffusion region 24 is formed in a separate step from the emitter region, if the impurity concentration is lowered too much, the contact resistance with the base electrode 16 increases to the point that the desired transistor characteristics are not obtained, and it becomes difficult to produce a transistor that can be operated at an input voltage lower than about 7 V. Also, even if the transistor is not used as a digital transistor, this structure yields a bipolar transistor with high switching speed and low power consumption, as mentioned above, but if the input signal voltage is too low, there will be a problem in that the transistor will not operate until the zener voltage of the reverse diode is lowered.
Moreover, when a digital transistor is produced, the input signals can vary widely with the user, and even if the voltage is set so that the built-in zener diode switches on the transistor, if the input signal has a high voltage and large current, the base current increases, and the amplified collector current also becomes extremely large, which can prevent the desired transistor characteristics from being obtained, or even break the transistor.